1. Field of the Invention
This invention relates to a technique for correcting positional deviation of ink dots during bi-directional printing using a printing apparatus capable of adjusting a platen gap.
2. Description of the Related Art
Recently, ink jet printers have become widely used as computer output devices. Some ink jet printers can perform so-called “bi-directional printing” to increase the printing speed.
A problem that readily arises in bi-directional printing is that of positional deviation of ink dots between forward and backward passes in the main scanning direction, which is resulted from, for example, backlash of main scanning driving mechanism and warping of a platen. As well known in the art, there is a technique for solving such problem of positional deviation, for example, as discussed in JP5-69625A disclosed by the present applicant. In this technique an amount of positional deviation (printing misalignment) is prestored so as to correct the dot positions during forward and backward passes based on the amount of positional deviation.
Several types of print media, such as regular paper and photo print paper, are available for inkjet printers. Each type of print medium has significantly different amount of deflection (referred to as “cockling”) due to absorption of ink. For this reason, a value of a platen gap has been set large enough to avoid contact between a print head and paper that is deflected due to the cockling. The setting of the platen gap to such a large value, however, undesirably increases influence of print head alignment on the ink dot positions on the print medium. Therefore, ink jet printers which can adjust the platen gap according to the type of print medium are recently proposed.
However, little consideration has been given regarding how to correct positional deviation of ink dots during bi-directional printing using a printer with adjustable platen gap.